The photon absorption technique is being utilized to measure bone mass in the radius. Females between the age of 30 and 90 are being followed in a prospective study to determine the rates of bone loss and the relationship of change in radial bone mass to the development of clinical osteoporosis. Preliminary studies are being undertaken to determine whether individuals on estrogen replacement therapy are losing bone at a different rate than those not on estrogens. The technique is being utilized to determine the affect of inheritance on bone mass. Monozygotic and dizygotic twins are being scanned for this purpose and initial analysis of the data indicate that there is a substantial genetic factor responsible for total bone mass, but genetic- environmental interaction plays a role. Similar studies are being undertaken in daughters of clinical osteoporotics with the goal of determining heritable factors in development of this clinical disorder. Efforts are also being pursued to further improve the technique utilized.